The tendency of optical fibers to leak optical energy when bent has been known since the infancy of the technology. It is well known that light follows a straight path but can be guided to some extent by providing a path, even a curved path, of high refractive index material surrounded by material of lower refractive index. However, in practice that principle is limited, and optical fibers often have bends with a curvature that exceeds the ability of the light guide to contain all the light.
Controlling transmission characteristics when bent is an issue in nearly every practical optical fiber design. The initial approach, and still a common approach, is to prevent or minimize physical bends in the optical fiber. While this can be largely achieved in long hauls by designing a robust cable, or in shorter hauls by installing the optical fibers in microducts, in all cases the optical fiber must be terminated at each end. Thus even under the most favorable conditions, certain bending, is encountered at the optical fiber terminals.
Controlling bend loss can also be addressed by the physical design of the optical fiber itself. Thus, ring features or trench features, or combinations thereof, to control bend loss are commonly found at the edge of the optical fiber refractive index profiles. See for example, U.S. Pat. Nos. 4,691,990 and 4,852,968, and U.S. patent application Ser. No. 12/583,212, filed Aug. 17, 2009, all incorporated herein by reference.
In a conventional graded index multimode fiber, high order modes suffer greater loss than low order modes when the fiber is bent. In bend-optimized multimode fiber designs (BOMMF), a trench is located at the outer edge of the graded index core to minimize the bend loss of high order modes. As is well known in this art, a trench refers to an annular region of depressed index located in, and part of, the cladding region.
A problem associated with BOMMF designs is that, while the trench reduces the bend loss of high order modes, it also changes significantly the propagation properties of high order modes. Dependent on the space between core and trench in the BOMMF designs, higher order modes travel either faster or slower than other modes. Thus the modal dispersion of higher order modes may be seriously distorted because of the unequal effect of the trench on the velocity of the propagating modes. This modal dispersion is often characterized by Differential Mode Delay (DMD) measurement. The challenge in the design of BOMMF with a trench is to maintain good bend loss performance while controlling the DMD of the high order modes. Ideally, an optical fiber has good bend loss performance and good DMD performance at the same time. In addition, the process to make such fibers should be robust and easy to control.
In typical optical fiber designs in which a trench is used to aid in controlling bend loss the trench is spaced from the edge of the alpha profile of the core by a shoulder. Typically the shoulder is silica with a refractive index of 1.456, but could be doped silica and have either a higher refractive index (positive delta n) or a lower refractive index (negative delta n) than silica. The width of the shoulder can be a design variable used in combination with other design parameters to modify fiber characteristics. An additional tool used to control DMD and bend loss performance is to truncate the edge of the core.